In optical technology there is a requirement for high power laser diodes with relatively stable single spatial mode bright light output characteristics. Brightness is defined as the optical output power per unit emitting area into unit solid angle from a spatially coherent source with a single lobed emission pattern. Brightness determines the minimum spot size to which light from a source can be efficiently focused. Accordingly, where efficient focusing of laser light is a requirement, for example, in cutting and welding of metal, brightness of the light output of a laser diode is an important property. Furthermore, the brightness of light outputted from a laser diode determines the extent to which the laser output may be coupled to an optical fibre of relatively small diameter. In general, in known high power laser diodes, an increase in optical power output can only be achieved at the expense of emission brightness or in the stability of the light output which in general results in increased beam wander. Single spatial mode waveguide laser diodes provide a relatively stable beam. However, due to the small emission aperture of such laser diodes, the maximum optical power which can be emitted is limited due to the possible onset of facet damage. Unfortunately, while simply increasing the emitting aperture permits higher optical power output, it also results in general, in loss of spatial coherence and brightness. Broad area laser diodes with relatively large emitting apertures which provide higher optical power output also suffer from the disadvantage of lasing in higher order modes and/or a decrease in spatial coherence as a result of the onset of filamentation, and thus the maximum brightness which would otherwise be achieved is limited. Nonetheless, even in broad area laser diodes the optical power output before the onset of optical damage to the laser diode is still limited by optical absorption at the facet. The requirement for spatial coherence in high optical power output laser diodes and the danger of optical damage in narrow aperture laser diodes have been limiting factors in the provision of high optical power output laser diodes for emitting a high intensity optical beam which can be focused to a micrometer size spot of the type typically required for efficient coupling to optical fibres for fibre amplifier applications, and for the production of high optical power densities for cutting, welding and soldering.
There is therefore a need for a laser diode and a method for providing such a laser diode which overcomes these problems.
The present invention is directed towards providing such a laser diode.